China's Minister of Information and Industry (MII) announced last Dec the "Technical Requirements and Test Method of Charger and Interface for Mobile Telecommunication Terminal Equipment" (YD/ T 1591-2006). Starting from the 14 of June 2007, all new mobile phones requesting for network access approval in China, will have to adopt the new battery charger interface standard described in the regulation. This is the government's effort to reduce the number of power adapters that become redundant due to the upgrade of equipment and thereby reduce the impact of the abandoned electronic parts on the environment and the waste of natural resources.
China is no longer just the largest mobile phone production country in the world; it is also a large consumer of new phones through new subscription and renewal. According to MII, China consumed about 100,000,000 units of handset in renewal each year and it is growing. Some initial saving may be transferred to the consumer, as they will not need to pay for a power adapter with each new handset purchased. Expecting the old, customized battery charger will be gradually replaced with new universal ones. This may even bring in new business opportunities for adapter makers.
As the battery charger is no longer equipped with special connector with different power characteristics customized to each mobile phone manufacturer. Instead, charger will be equipped with a standard USB Type A plug and the output power is regulated to ensure that it can be used across all new handsets. Battery charger that is compliant to the new regulation should request for the certification from China Telecommunication Technology Laboratory (CTTL) of MII. As of 31 May 2007, CTTL has already certified 15 mobile phone charger models from 14 companies, which consist of mostly Chinese phone suppliers. For non-Chinese phone suppliers with an international market, their chargers are standardized for different phone models selling in different countries, it may take them some more time to change it.
Electrical characteristics of USB charger interface
The new regulation requires all handsets to provide a USB interface for battery charging and data transmission purpose. It is however flexible for the phone vendor to decide on the interface on the handset itself. If it is not compliant to the USB type A connector standard, an adapter cable with USB connectivity will have to be provided. With this USB interface in place, not only handsets could be recharged by universal chargers; mobile phone can also be recharged by other USB hosts such as notebook PC. A great news for business travelers in China, as they can slightly lighten their baggage loading.
The output voltage characteristic is defined by the new regulation. The nominal output voltage of the charger should be 5 V with a +/-5% tolerance; that means, the input voltage to the mobile phone connector should remain between 4.75 V to 5.25 V. Moreover, a handset should limit its maximum input current to 1.8 A. The Lithium-ion or Lithium-polymer battery pack rating remains around 900 mAh today, thus requiring a charge current of less than 1A. However, to set a higher input current limit to 1.8A leaves room for development. As the charge current is expected to increase in the foreseeable future, as new battery pack with higher capacity will be needed to support the power consuming multimedia functions in new 3G handsets.
As mentioned before, a USB host can be connected to the USB interface on handset either for battery charging or data transmission purpose. How can the handset distinguish a USB port from a power adapter and be able to select the right input current limit? The regulation requires that the power adapter to have both D+ and D- line be shorted together inside the charger. When the data lines are not shorted, the handset will recognize that the source is a USB port and thus will limit the input current to maximum 500mA in order to conform to the USB standard.
Requirement on the internal charging circuit in mobile phone
Although we usually refer to the power adapter as 'charger,' the charging circuit is actually locating inside the mobile phone. Given that the minimum voltage supplied by the adapter to be 4.75 V, and that the maximum voltage of a Lithium-based battery is 4.2 V. That leaves voltage headroom of 0.55 V between the power input (Vin) and the battery. 0
As shown in Figure 1, the charging is controlled by the PMU (Power management unit) of the terminal and the MOSFET acts as a pass element for the charging current. A Schottky diode is connected between the MOSFET and battery in order to cut off any reverse current leakage path through the body diode of the MOSFET when it is turned off.
Figure 1. A typical charging circuit controlled by a power management unit in a mobile phone.
Let's calculate the voltage dropout over the 2 pass elements--MOSFET and Schottky diode in this charging circuit:
Vdropout = Charging current x Rds(on) of MOSFET + Vforward of Schottky diode
= 1A x Rds(on) + Vf
Assume typical Vf value to be 0.35V and Vdropout to be less than 0.5V (in order to stay within the 0.55V headroom), we need a MOSFET with Rds(on) less than 150 mΩ ( or we could say the dropout voltage over MOSFET must be less than 0.15V)
As we can observe the Schottky diode contributes a very significant dropout voltage of 0.35V. As the charge current increases, this may soon become a blocking point. The dropout over the pass components can be reduced by replacing the Schottky diode with a transistor with low VCE (Sat) or a MOSFET with low Rds(on) in order to fit into the narrow headroom imposed by the USB charger standard. Some pass component solutions are shown in below diagrams:
Figure 2. NSS12600CF8T1G is a low VCE(Sat) (0.1V) transistor that can be used as a pass element in the charging circuit.
Figure 3. Using dual low Rds(on) MOSFETs as pass elements. NTHD4102P, the dual P-Channel MOSFET with Rds(on) per switch at typical 64Ω, thus generate only a voltage dropout of 0.13V with 1 A charging current.
Risk of damaging handset
Apart from the conveniences, standardizing charger interface implies that any power source with a USB plug can be connected into a new handset, no matter what their output power is. As USB plug becomes a common interface, it is not only limited for use on mobile phone, but also for other portable electronics, like portable DVD. In that case, the adapter output voltage will be 12 V instead of 5 V. Even if a power adapter is designed to output 5V, if its regulation is poorly designed, it may output a voltage with wider tolerance than the 5% imposed. To protect portable terminal these over-voltage conditions, MII imposed mobile handset to integrate the over input voltage limit in the charging control circuit. Whenever the input voltage from charger exceeds 6 V, the over-voltage protection circuit should be activated to isolate the faulty power source from the rest of the circuitry in handset.
Over-voltage protection IC
The Over-Voltage Protection (OVP) IC is an integrated circuit that can operate up to a very high voltage like 28V without being damaged. It contains a voltage detector and an internal driver circuit to control a MOSFET as a switch to isolate the power supply from the system whenever the input voltage is above the over-voltage detection threshold (this threshold voltage is sometimes referred to as Over Voltage Lock-Out, OVLO). NCP347MTAE is an OVP dedicated to protect portable terminals from USB charger. It has a built-in 50 ms start-up delay, during this time the internal switch remains open, so that no high level transient voltage will pass onto the system. During the operation the MOSFET will be turned on, and it can be triggered to turn-off in 1.5 μs (typical) when a fault situation is detected. When selecting OVP IC for USB charger, one should make sure that the maximum OVLO value to be below the "absolute maximum operating voltage" of the ICs connecting directly to the power supply. To meet the USB charger requirement of MII, an OVLO value of just below 6V will be suitable. As discussed before, due to the high charging current and limited voltage headroom, the Rds(on) of the pass element (the MOSFET) should remain as low as possible. In NCP347, the typical Ron of the integrated N-channel MOS switch is only typical 65 m Ω. It is known to be the lowest for the same kind of product in the market.
Figure 4. A typical application of NCP347 with a Lithium-ion battery charger IC.
OVP IC with interface to the processor allows the implementation of smarter protection scheme. Given a /Flag pin to report faulty situation and a /Enable pin, one can program the processor to definitely disable NCP347 when a continuous faulty situation is detected. Otherwise, the OVP should be turned-on automatically again when the faulty situation is removed. Finally, its ultra thin package of 2.5x2.5x0.55mm should fit into any slim, portable design. This kind of protection circuit will ensure the handset to pass the 60 minutes over-voltage test condition requested by CTTL.
The new battery charger interface standard has been imposed by the MII and so we should expect all new handsets to be released in the Chinese market to be equipped with a USB connector, in order to connect to the standardized battery charger. Potentially this will reduce up to 100 million of redundant battery charger that would be sold with the new handsets in China each year. This new standard will bring in saving in the total handset material cost and reduction in pollution caused by the abandoned electronics.
The new standard will bring in more convenience to the end users, as it will be easy to find a compatible charger in public places like office or hotel. User can also make use of other USB host like notebook PC to recharge the phone battery. Given the output voltage supplied by the USB charger will leave very little voltage headroom between the charger input and the battery. One should select the pass elements with the lowest Rds(on), in order to reduce the voltage dropout during high current charging. Both dual MOSFETs and transistor with low Vce(sat) are suitable solution for such charging solution. The standard interface also introduces the risk of having a non-standard charger to be connected to a handset. MII therefore required the phone vendor to include an over-voltage protection circuit inside the handset. OVP ICs are new type of protection circuits that provide an all round protection solution to handset. It can be triggered very rapidly in within micro-seconds to isolate any surge voltage from the phone system.
About the Authors
Crystal Lam is the product marketing manager at ON Semiconductor in charge of the audio amplifier and LED driver portfolios focusing on the development of new analog integrated circuits for the mobile phone and portable consumer markets. Before joining ON Semiconductor, Crystal worked as a technical marketing engineer at Motorola, and a strategy consultant at the Monitor Group. Crystal holds an BSEE degree from the University of New South Wales, Australia and an MBA degree from the ESSEC business school, France. She can be contacted at Crystal.Lam@onsemi.com.
Harry Liu is the Senior Field Application Engineer at ON Semiconductor, responsible for DC-DC products technical supporting. Prior to joining ON Semiconductor in 2003, he worked as a R&D manager in consumer products development. He can be contacted at firstname.lastname@example.org.